The identification of viable myocardium is an important, common clinical goal, e.g., in the evaluation of patients with chronic ischemic disease for possible revascularization therapy (Thornhill et al., “The assessment of myocardial viability: a review of current diagnostic imaging approaches,” J. Cardiovase. Magn. Reson. 4:381-410 (2002)). Viability imaging refers to imaging techniques that identify abnormal myocardium which could potentially recover function after reperfusion (viable myocardium). Magnetic resonance imaging (MRI) is increasingly used to identify viable myocardium (Sandstede, “Assessment of myocardial viability by MR imaging,” Eur. Radiol. 13:52-61 (2003)). The standard MRI evaluation is based on a comparison of separately acquired images of the left ventricle—a delayed enhancement MRI (DE-MRI) image and a set of cine-MRI image frames. The cine-MRI image frames provide information on whether the myocardium is moving abnormally. But, abnormal motion can be the result of tissue that is chronically infarcted (non-viable) myocardium that is being passively moved, or abnormally contracting myocardium (viable) that could potentially recover function after revascularization. The DE-MRI image highlights chronic scar with bright signal, helping to determine non-viable myocardium from viable myocardium.
The delayed enhancement MRI (DE-MRI) image is typically acquired in the diastolic phase (when little heart motion occurs) at a delayed time after injection of a contrast agent (e.g., gadolinium chelate) and is typically acquired as a single two dimensional (2D) slice. DE-MRI uses an inversion recovery (IR) pulse to null normal myocardial signal (Simonetti et al., “An improved MR imaging technique for the visualization of myocardial infarction,” Radiology 218:215-223 (2001)). This creates a tissue contrast where non-viable myocardium usually appears bright (Kim et al., “Relationship of MRI delayed contrast enhancement to irreversible injury, infarct age, and contractile function,” Circulation 100:1992-2002 (1999)). DE-MRI images are often obtained in a single breath-hold. DE-MRI can show sub-endocardial and other non-transmural infarcts (Wagner et al., “Contrast-enhanced MRI and routine single photon emission computed tomography (SPECT) perfusion imaging for detection of subendocardial myocardial infarcts: an imaging study,” Lancet 361:374-379 (2003)), the identification of which has prognostic value.
Cine-MRI image frames are typically a set of single 2D slices acquired throughout the cardiac cycle, which can then be viewed in cine-mode. A single breath-hold is usually used. While non-tagged versions are often used alone, tagged versions show subendocardial contractility better and help discern true from apparent wall thickening due to through-slice motion. Both types of cine-MRI have an inherently low tissue contrast, however, so that non-viable myocardium is difficult to identify.
Image information from DE-MRI, which highlights non-viable myoeardium, and cine-MRI, which demonstrates myocardial wall motion, are used together to make the primary distinction between viable and non-viable myocardium. Abnormal myocardial wall motion with transmural hyper-enhancement indicates non-viable myocardium while abnormal myocardial wall motion with no hyper-enhancement represents viable tissue (ischemic, hibernating, stunned, or myopathic), the sub-types of which may be distinguished with first-pass perfusion MRI (rest and/or stress), stress cine-MRI, and clinical history. This standard MRI approach has been successfully validated against positron-emission-tomography (PET) (Klein et al., “Assessment of myocardial viability with contrast-enhanced magnetic resonance imaging-comparison with positron emission tomography,” Circulation 105:162-167 (2002)) and single-photon-emission-computed-tomography (SPECT) (Wagner et al., “Contrast-enhanced MRI and routine single photon emission computed tomography (SPECT) perfusion imaging for detection of subendocardial myocardial infarcts: an imaging study,” Lancet 361:374-379 (2003)) for the assessment of myocardial viability.
Both the cine-MRI and DE-MRI images can be acquired using a segmented process, in which different segments of the k-space of each image are acquired from different cardiac cycles but at approximately the same phase in each of the cycles. This allows a higher resolution image to be produced, as only a segment of the image must be captured from each cardiac cycle.